Trow . — The Karyology of Saprolegnia. 619 
(Agardh, 1824), Cornu, was found very unexpectedly, appa- 
rently associated with no other species, in a test experiment 
as to the purity of the Cardiff Water -Works water. I have 
seen most of the forms (or species) of the Ferax-group of 
De Bary, but have not been able to study them critically. 
Fixing. Most of the better known methods of fixing were 
tried, viz. solutions of osmic acid, picric acid, and chromic 
acid, absolute alcohol, as well as a hot saturated solution of 
mercuric chloride — the last being recommended by Hartog 
and found most satisfactory by Humphrey. I have found 
Hartog’s method to be the best, and have used absolute 
alcohol side by side with it, as a means of avoiding any error 
due to the effect of the fixing liquid. 
Imbedding and Sectioning . The fixed material was gene- 
rally washed for twenty-four hours in running water, and in 
the earlier experiments was passed into absolute alcohol by 
means of Schultze’s dehydrating apparatus, but later on this 
was dispensed with. Graded alcohols give results which are 
sufficient for all practical purposes. The dehydrated material 
was passed into paraffin, chiefly through the medium of xylol, 
but sometimes of chloroform. The position of the fly and 
fungus is well seen in the paraffin. The fly was generally 
bisected longitudinally, certain useless portions — determined 
by preliminary experiment — cut away, and the remainder 
sectioned by means of a Jung microtome. Some of the 
sections were of 7-5 n thickness, a few of from 2 to 3 jx, but 
most of them of 5/ot. For the more difficult work, the last- 
mentioned thickness is very convenient : one gets by this 
means about four sections through the young oospore. I may 
add that the chitinous coat of the fly is easily sectioned, but it 
is better to cut it away so as to save the edge of the knife 
and avoid the sectioning of useless material. Moveover, 
the smaller the surface of section, the longer becomes the 
series mounted on the same slide — an advantage of some 
importance. 
Staining. In the earlier experiments an attempt was made 
to stain the material en masse with haematoxylin. The 
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